1. Field of the Invention
The invention is in the field of mixing valves for mixing a plurality of fluids in predetermined proportions.
2. Description of the Prior Art
In some fluid applications it is required that two separate fluids be mixed immediately prior to the application. An example of this is in fire fighting vehicles where a fire retardant liquid such as "Light Water" or protein foam is mixed in small proportion with water immediately prior to being sprayed or jetted into a fire. In such mixtures the fluid proportioning is generally required to be quite accurate for optimum effectiveness, and in an endeavor to provide accurate proportioning of the ingredients it is common practice in the art to employ a mixing valve into which both fluids flow and which is designed to mix the fluids in a given proportion.
The type of mixing valve conventionally employed in fire fighting equipment embodies a spring biased piston wherein the extent of displacement of the piston increases according to the volume of flow through the mainstream, increased piston displacement allowing a greater amount of the fire retardant material to flow into the mainstream. A serious disadvantage of this type of mixing valve is that it is inherently limited to only a single percentage mixture of the liquid ingredients, so that only one weight or density of fire retardant material can be used in the equipment. Fire retardant liquids are made by several different companies, but each of these liquids has a different weight or density, whereby fire fighting equipment employing a conventional mixing valve is limited to use of only a specific brand of fire retardant material. Specifically, the fire retardant liquids produced by the different manufacturers are adapted for either a 3%, a 6%, or a 10% mixture with water, and with conventional mixing valves each of these percentage mixtures requires a different mixing valve to produce the required proportioning.
While a variety of adjustable mixing valves is found in the prior art, none of these is adaptable for adjustment to specific predetermined percentage increments of proportioning where each of the predetermined percentages will remain substantially fixed regardless of wide variations in the mainstream flow volume. Also, such prior art mixing valves tend to be undesirably complex and expensive, and generally are not suitable or reliable for handling the large flow volumes that are required in fire fighting equipment.
Examples of some of these prior art adjustable mixing valves are found in Sanderson et al U.S. Pat. No. 3,934,604 which discloses a mixing valve where an eccentric is used to adjust mixing proportions by adjusting orifice size; Hirst et al Patent No. 2,480,925 which discloses a mixing valve where primary proportioning is accomplished by lever rotation of a piston assembly, and where the piston is also rotated by camming action as it is displaced so that proportioning adjustment is a function of flow rate; and Kanelos U.S. Pat. No. 2,751,920, Chaffee U.S. Pat. No. 2,471,142 and Lindsay U.S. Pat. No. 3,647,002, all of which disclose mixing valves which employ adjusting screws for gradually varying the amount of displacement of a valve member.